I have not toured yet but have read many related books and journals and I don't recollect truck stops mentioned as a place to get a shower.
A relative owned a truck stop and he indicated that truck stops have paid showers.
Another question is that when stealth camping, one cannot shower/bathe unless next to a river or lake and weather permitting. I am habituated to shower every morning and and am uncomfortable with the idea of starting a day w/o a shower. I am sure many bike tourists have experienced the same and wonder how to manage. Guess such luxuries have to be left home!